1. (Field of the invention)
This invention relates to a starting electric motor for starting an engine, and more particularly to an improvement of the pinion shafting structure in an overhang type starting electric motor in which a hollow movable pinion cylinder engageable with a ring gear of an engine is moved axially in the front bracket.
2. (Prior art)
FIG. 1 is a sectional view showing the pinion shafting structure in a conventional starting electric motor disclosed, for instance, in Japanese Utility Model Application (OPI) No. 6679/1986 (the term "OPI" as used herein means an "unexamined published application").
As shown in FIG. 1, a rotary output shaft 1 of the motor includes a spline part 1a having a helical spline, and a small diameter part 1c. An overrunning clutch 10 movable on the output shaft 1 back and forth comprises a movable pinion cylinder 2, a clutch outer 8, and rollers 9. The movable pinion cylinder 2 comprises; a clutch inner part 2a; a cylindrical part 2b slidably fitted in a bearing 4 which is fixedly fitted in a front bracket 12; and a pinion part 2c engageable with a ring gear 20 of an engine.
Further in FIG. 1, reference numeral 5 designates a dust protective cap; 6 and 7, parts forming a stopper structure for the movable pinion cylinder 2; and 3, oil-impregnated bearings fitted in the movable pinion cylinder 2 in such a manner that they are located at the cylindrical part 2b and the pinion part 2c.
The operation of the starting electric motor thus constructed will be described. Upon application of voltage to a magnet switch (not shown), a plunger is attracted, so that the overrunning clutch 10 is moved forwardly through a lever. As a result, the movable pinion cylinder 2, being engaged with the overrunning clutch 10, is engaged with the ring gear 20 of the engine, while the contact means (not shown) of the magnet switch is closed to supply current from a battery. Consequently, an armature of the DC motor is started, and the ring gear 20 of the engine is driven. At the same time, the front end of the movable pinion cylinder 2 abuts against the stopper 6 and the movable pinion cylinder 2 is stopped.
The conventional starting electric motor is constructed as described above. Therefore, the grease applied to the oil-impregnated bearings 3 are liable to be moved along the armature rotary shaft 1 forwardly or backwardly, with the result that the service lives of the bearings 3 are shortened as much. Furthermore, the conventional starting electric motor suffers from the following problems: In the case where it is required to extend the rotary shaft 1 through the pinion part 2c of the movable pinion cylinder 2 with the tooth form of the pinion formed at the pinion part made smaller or with the number of teeth reduced, either the thickness of the teeth base of the pinion or the diameter of the rotary shaft must be reduced, with the result that the mechanical strength thereof is unavoidably reduced. On the other hand, the clutch inner part 2a and the clutch outer 8 become out of alignment, thus shortening the service life of the overrunning clutch.